1. Field of the Invention
The present invention relates to an optical disc verification method and an optical disc medium that would contribute to high-speed writing.
2. Description of the Related Art
Examples of various storage media from/on which data can be read and/or written optically (which will be simply referred to herein as “optical discs”) include write-once discs such as CD-Rs, DVD-Rs and BD-Rs, to which data can be just added, and rewritable discs such as DVD-RWs, DVD-RAMs, and BD-REs. A standard writing speed is defined for all of these optical discs. More specifically, the writing speed is represented as either a linear velocity or a transfer rate. In this description, the writing speed will be represented herein by the linear velocity in most cases.
Recently, in response to the demand from the market and thanks to development of technologies, the highest writing speeds on optical discs have been increasing so rapidly that a write operation can now be performed on DVD-Rs at as high a writing speed as 16×. As for BD-Rs, on the other hand, discs and recorders that realize 4× writing are already used extensively. And optical discs that will be able to get writing done at even higher speeds are also under research and development. As used herein, the “**× speed”, for example, means that the writing speed is ** times as high as the standard writing speed.
However, the higher the maximum writing speed of an optical disc, the stricter the writing conditions should be. That is why the manufacturers and verification authorities of optical discs test a given optical disc to see if its properties meet predetermined criteria for high speed writing. For that purpose, a test write operation is actually performed on inner, middle and outer tracks of a sample optical disc to see if the signal quality index value of the read signal falls within a predetermined range. And only optical discs that have passed such a test are shipped as products.
In a situation where a high-speed write operation is performed on a normal optical disc with a thickness of 1.2 mm and a diameter of 12 cm, the optical disc is preferably used at rotational frequencies of approximately 10,000 rpm or less according to the current state of the art, in view of the maximum allowable rotational frequency of a spindle motor, the noise, and safety measures to take to avoid the rapture of the disc. Also, if an optical disc were rotated at a constant rotational frequency, the write linear velocity on an inner area with a relatively small radius would be less than the one on an outer area with a relatively large radius. For that reason, if the highest writing speed were more and more increased, then the rotational frequency would be too high on inner tracks with a relatively small radius to get the data written at desired high speeds easily. More specifically, in a BD, for example, if the writing speed is 5×, then the rotational frequency will reach approximately 9,800 rpm in the vicinity of a radial location of 24 mm. And if the writing speed is further increased to 6× or more, then the rotational frequency in the vicinity of the radial location of 24 mm will exceed 12,000 rpm.
That is why even if an optical disc has been designed and manufactured so as to meet predetermined properties for high-speed writing, it is impossible to see, by the conventional optical disc testing method, if its signal quality index value on inner tracks, where the rotational frequency of the optical disc increases significantly, falls within the predetermined range for high-speed writing.
Also, even if an optical disc apparatus for use in the testing process had a spindle motor that is powerful enough to achieve a rotational frequency exceeding 10,000 rpm, the spindle motor and the optical disc would produce such great vibrations at those high frequencies that the tracking servo would lose stability too much to get the given optical disc tested accurately. That is why in the prior art, good signal quality cannot be ensured when a high-speed write operation is performed on inner tracks on an optical disc.
For these reasons, even if a drive that could perform a high-speed write operation on inner tracks with good stability was developed, an optical disc medium, which could not ensure good signal quality when a high-speed write operation was performed on inner tracks, would still have to be used.